Biceps Reflex Status in Biceps Tendon Rupture
The biceps reflex is typically preserved in biceps tendon rupture, whether proximal or distal, because the reflex arc remains intact through the muscle belly despite tendon discontinuity.
Understanding the Clinical Anatomy
The biceps (C5-C6) reflex tests the integrity of the spinal reflex arc, which involves:
- Sensory input from muscle spindles within the biceps muscle belly
- Afferent transmission through the musculocutaneous nerve
- Spinal cord integration at C5-C6 levels
- Efferent motor response back through the musculocutaneous nerve to the muscle
The critical point: The reflex arc depends on intact innervation and viable muscle tissue, not on tendon continuity at either end.
Why the Reflex Persists
In both proximal and distal biceps tendon ruptures:
- The muscle belly remains innervated - The musculocutaneous nerve supply to the biceps brachii is unaffected by tendon rupture
- Muscle spindles remain functional - These proprioceptive organs within the muscle continue to respond to stretch
- The reflex pathway is preserved - The neurological circuit from C5-C6 through the peripheral nerve remains intact
The biceps has two heads (long and short), and rupture patterns vary. Partial ruptures with an intact short head are most common 1, and even in complete ruptures, the muscle belly retains its reflex capability despite loss of mechanical continuity at the tendon insertion.
Clinical Implications
Do not rely on reflex testing to diagnose biceps tendon rupture. The physical examination should focus on:
- Visual inspection - Proximal muscle retraction creating a "Popeye" deformity (more obvious in proximal ruptures)
- Palpation - Absence of the distal biceps tendon in the antecubital fossa for distal ruptures
- Strength testing - Weakness in elbow flexion and particularly supination (distal ruptures cause significant supination deficits) 2
- Specific provocative tests - Hook test for distal biceps integrity
Diagnostic Approach
When biceps tendon rupture is suspected clinically:
- MRI is the gold standard for confirming diagnosis and characterizing tear patterns (partial vs. complete, proximal vs. distal) 1
- Ultrasound is operator-dependent but can identify complete distal ruptures, though it underperforms for partial tears 1
- FABS view MRI (flexion-abduction-supination) provides optimal visualization of the distal biceps tendon 1
Common Pitfall
The preserved biceps reflex can falsely reassure clinicians that no significant pathology exists. A normal reflex does not exclude biceps tendon rupture. The diagnosis must be made through careful inspection, palpation, and functional testing, supplemented by imaging when indicated 2, 3.
In the rare scenario of concomitant brachial plexus injury, biceps rupture becomes particularly challenging to diagnose due to the paretic limb, and intraoperative palpation may be necessary 4.